Universal programmable logic control device have been developed to serve as circuit breakers in order to protect electrical circuit and devices from damage due to circuit overloads and ground faults and are generally part of safety related equipment. In the event of a fault, the circuit is broken for safety reasons.
For example, U.S. Pat. No. 4,866,557 to Fitts et al. (“'557”) discloses a low level voltage programmable logic control device for monitoring the current of a circuit and for signalling a control device, the programmable logic control device including electrical power circuitry for activating the control device and electrical circuitry for monitoring the current, the electrical circuitry including: adjustable electrical signal input circuitry (in the form of ampere taps) to match the programmable logic control device to the electrical current in said circuit, adjustable pick-up circuitry for adjusting the level of the current monitored in the electrical circuit, and adjustable time circuitry for adjusting the time duration of the selected level of electrical current, so as to command the electrical power circuitry to activate the control device when the electrical current in the electrical circuit reaches the selected level and time duration and means for controlling the level of voltage applied to the adjustable pick-up circuitry and time circuitry substantially without resistive burden, said means passing a reference voltage which is less than an input voltage to said means.
The '557 programmable logic control device is described as universal in the sense that one device can be used to effectively monitor electrical systems over a range of electrical characteristics by selecting the appropriate ampere tap and control knobs which are incorporated in the device rather than utilizing a number of current transformers or logic devices. In the preferred embodiment disclosed therein, the programmable logic control device may be universally utilized to monitor currents between a range of 30 to 4400 amps.
Other arrangements are shown in U.S. Pat. Nos. 6,707,651 B2, 4,949,362, 4,733,323 and 4,866,557. In some circumstances these circuit breakers have experienced spurious tripping that can be due to a number of factors such as short-duration, high amplitude current transients causing unwanted tripping of some circuit breakers. These high amplitude current transients can include high frequency electrical noise spikes from lightning storms or the like.
Generally speaking spurious tripping of circuit breakers is undesirable due to unnecessary power loss and erratic safety functions of the circuits. Spurious tripping concerns have been particularly documented in the nuclear industry as evidenced by the Sep. 17, 1993 NRC Information Notice 93-75 entitled “Spurious Tripping of Low-Voltage Power Circuit Breakers with GE RMS-9 Digital Trip Units” (U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Washington, D.C., especially when these devices are implemented in nuclear reactors.
Nuisance or spurious tripping should be understood as any event where a power circuit breaker undergoes a false trip indication due to a transient. This can occur when, for example, there is a ground current surge during a lightening strike. Further, any electric circuit operating in harsh environment applications routinely face temperature fluctuations, and these fluctuations can often be severe. Moreover, in the nuclear reactor context, circuits may be exposed to radiation which can erode the integrity of the circuit components and jeopardize their performance.
In light of the foregoing, what is needed is a programmable logic control which is more stabile to temperature variations. What is further needed is a programmable logic control which is more resilient to transient spikes. What is yet further needed is a programmable logic control which is less susceptible to radiation.